The invention relates to a color cathode-ray tube (CRT) and, more particularly to a color CRT including a luminescent screen assembly.
Lacquer filming is an important process in the manufacture and operational performance of a cathode-ray tube (CRT) because the surface character of the lacquer film dictates the quality of the aluminum film, thereby having an impact on CRT light output. The lacquer film should be smooth and continuous, otherwise perceptible discolorations and brightness non-uniformities could be observed during CRT operation.
When the lacquer film is smooth and continuous, the aluminum will likewise be smooth and continuous, enhancing light output and visual uniformity in an operating CRT. During operation, electrons must penetrate through the aluminum and the aluminum must reflect any phosphorescence toward the viewer.
During manufacturing, the aluminum film must also behave as a semi-permeable membrane. Essentially the aluminum film must allow gaseous species to pass therethrough during the screen bake process, a necessary thermal process employed to remove remnant organic materials from the screening process. These species must pass through the aluminum without causing the aluminum film to bubble, peal or blister. Typically CRT manufacturers will apply microscopic crystals of boric acid or ammonium oxalate to the lacquer film substrate prior to aluminizing the screen, thereby creating microscopic perforations in the aluminum film. These perforations permit the gases to diffuse through the aluminum film while allowing the aluminum film to retain light-reflecting character.
One means of applying thin films onto the interior surface of panel of a CRT is an electrostatic spray technique. U.S. Pat. No. 5,807,435, issued on Sep. 15, 1998, describes such a technique. This has been an efficacious method of filming a CRT luminescent screen having phosphor stripes printed using an electrophotographic screen process (EPS). An EPS process has been described in U.S. Pat. No. 5,474,866, issued on Dec. 12, 1995, wherein a layer of a suitable organic photoconductor (OPC) is applied to a layer of an organic conductor (OC) on the interior of a panel followed by: suitably charging OPC; selectively discharging appropriate areas of the OPC by irradiating the OPC with light through a shadow mask positioned within the panel; removing the mask and depositing triboelectrically charged phosphor of a color to form the first color element for the CRT; and repeating the charging, exposing and depositing steps for each of the remaining two colors.
The phosphor stripes are then fixed after the EPS process to prevent the phosphor particles from substantially moving during the filming process. Simultaneous fixing of the three color phosphor stripes is achieved by applying a gentle electrostatic spray of a solvent or solvents onto the panel, wherein the OPC is dissolved and encapsulates the phosphor particles. As the solvent molecules evaporate, the phosphor particles become effectively anchored (or often referred to as fixed).
Prior to the deposition of film lacquer by electrostatic spraying or spin coating, the fixed phosphor substrate is rough and discontinuous. With the proper application of the lacquer film onto the fixed screen, the resultant substrate, at the air-lacquer interface, is smooth and continuous; consequently, the aluminum, which is vacuum evaporated onto the lacquer film, is smooth and continuous. The panel assembly is later screen baked to remove the organic materials; hence, the panel is ready for tube finishing.
Two risks associated with the lacquer film are the potential for the aluminizing layer on top of the lacquer film to blister during the screen bake process, if the mass of the organic materials is too high, and the potential for the light output of the CRT to be too low because of the roughness of the aluminum surface. Rough aluminum occurs if the lacquer film is too low in screen weight or the lacquer film is not smooth and continuous. The challenge is to employ lacquer film formulations which can reduce the risk of aluminum blistering and yet allow the aluminum film to be smooth and continuous such that the light output is enhanced.
The present invention relates to the manufacture of a cathode-ray tube having deposits of phosphor powder on the interior of a glass panel, wherein a novel film formulation is applied to the deposits of phosphor to form a filmed surface followed by the application of a thin layer of aluminum on the filmed surface and the exposure of the glass panel to a sufficiently high temperature to volatilize the organic components. The novel filming formulation has at least one lacquer material and at least two solvents, wherein one of the solvents has poor solvating power for at least one lacquer material.